The present invention relates to a helical screw rotor compressor that comprises a rotor housing which includes a barrel wall between two parallel end walls and further includes an input port at a first end and an outlet port at a second end and which has internally the form of two parallel, mutually intersecting cylinders. The compressor also comprises two rotors which co-act with one another and also with the rotor housing, said rotors including a rotor shaft which is mounted in the end walls, and a rotor body which surrounds the shaft in said rotor housing with parallel end surfaces adjacent the end walls of the rotor housing. The rotor bodies include mutual discrete helical lobes which each have a crown, a first or leading side surface on a first side of the crown and a second or trailing side surface on a second side of the crown.
Such compressors are well known to the person skilled in this art.
In recent times, rotors for screw compressors have increasingly been produced from a metal shaft around which there has been anchored a polymeric body that includes helical lobes separated by intermediate grooves. Such rotors are described in WO 01/28746 and in WO 01/28747 for instance. These polymer bodies have planar parallel end surfaces that face at right angles to the metal shaft. Because the lobes extend helically, a first side surface or flank surface of the lobe defines an acute angle at one end surface and a second side surface or flank surface of said helical lobe defines an obtuse angle with said end surface. The thickness of the lobe material is relatively small in the region in which the first side surface of the lobe defines an acute angle with said end surface, resulting in a comparatively weaker lobe. This is probably the reason why pieces of the lobes of the rotor body are torn loose when the rotors are used as active components in helical screw compressors. This applies in particular to that end of the rotor at which the highest pressure prevails, in other words at the outlet port of the compressor. Damage of this nature leads to a reduction in compressor efficiency. This may be due to a connection between an outlet space on the high pressure side of the compressor and its high pressure chamber being opened earlier than intended, therewith allowing gas to flow from the outlet space into the compressor chamber under certain conditions. Torn-off fragments, i.e. chips, slivers etc., also result in contamination of the gas system and in the worst case in significant damage to or even destruction of the compressor. Such damage occurs to a small extent, when the rotor is made of a metal that is much stronger and less brittle than polymeric material.